Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of engine speed to driveshaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Many automatic transmissions implement a discrete number of different transmission speed ratios in which each ratio is establish by engaging a particular subset of clutches. A clutch that selectively holds a gearing element against rotation may be called a brake. Some clutches may be actively controlled devices such as by hydraulically actuated multi-plate wet clutches. Other clutches may be passive devices such as one way clutches. To shift from one speed ratio to another speed ratio, one clutch is engaged and another clutch is released. A shift to a lower speed ratio is called an upshift. An upshift may occur in response to an increase in vehicle speed or in response to a decrease in driver demanded power. A shift to a lower speed ratio, on the other hand, is called a downshift. A downshift may occur in response to a driver demand for more power, or in response to the vehicle slowing down.
During a shift, the vehicle speed changes only slightly, but engine speed may change significantly. The change in engine speed is often opposite to the trend before and after the shift. For example, as a vehicle accelerates in a given speed ratio, engine speed gradually increases. However, during an upshift, the engine speed decreases. The torque exerted at the transmission output during a shift varies both because the torque ratio changes and because torque is either diverted to speeding up the engine or is generated by slowing the engine. Drivers and other vehicle occupants have expectations regarding output torque and engine speed and are dissatisfied when the actual output torque or engine speed behaves differently than expected.
Although the normal flow of power is from the engine to the wheels, power may flow in the opposite direction when the vehicle is coasting. Some power is needed to keep the engine rotating. If the engine generates less than this amount of power, the power is supplied by the transmission. Vehicle occupants experience this as slight increase in deceleration rate compared to how quickly the vehicle would slow down with the transmission in neutral. When a downshift occurs during coasting, the engine speed increases. If the energy to increase the engine speed is provided by the transmission, vehicle deceleration may increase noticeably during the downshift contrary to vehicle expectations. Unfortunately, the torque capacity of the oncoming clutch strongly influences both the output torque and the rate of change of engine speed, complicating the task of matching both output torque and engine speed to driver expectations.
Many transmissions are controlled by an electronic controller. The controller sets parameters such as the pressure provided to various clutches. The parameters set by the controller influence a number of characteristics observable by vehicle occupants, such as output torque and engine speed. The objective of control algorithms is to set the controlled parameters such that the observable characteristics have desired values. In a closed loop control scheme, the controller utilizes a measurement of the observable characteristics as feedback signals, adjusting the controllable parameters in response to differences between the measured quantities and the desired values. A system with multiple controllable parameters and multiple feedback signals is decoupled if changes in each control parameter influence only one feedback signal. Decoupled systems are easier to control than strongly coupled systems because each controlled parameter can be set independently of the others. In a semi-coupled system, the controlled parameters can be set in an order such that changes in later parameters do not influence the feedback signals used to set the earlier parameters.